According to the invention, these new biomaterials are characterized in that they comprise at least one compound consisting of an association of collagen, chitosan and glycosaminoglycans.
Chitosan, preferably obtained by simple deacetylation of chitin, remains acetylated at a degree of acetylation between about 10% and about 40%.
The biomaterials according to the invention can be used in all losses of substance and for reconstitution of all tissues.
Actually, all the physical characteristics (resistance, elasticity, size of pores, degradation time. . . ) can be modified and adapted to the planned use by acting on the proportion of the various constituents.
When implanted, the extraceilular matrix constituted by this biomaterial will be colonized by adequate cells which will reconstitute IN VIVO the tissue which is specific to them.
Thus, osteoblasts will synthesize a connective tissue which will calcify to give bone. Fibroblasts will give connective tissues (dermes, arteries. . . ) and nerve cells, nerves.
This invention will first be described in one of its most important applications which is artificial skin allowing the simultaneous reconstitution of the dermis and epidermis, while avoiding the formation of hypertrophic cicatrices.
This entirely biodegradable artificial skin, will be used mainly for early covering of burns after early excision of cutaneous wounds of man and animals.
It is known that in the case of deep, extensive burns, it is essential to reconstruct as fast and lasting as possible an impermeable cutaneous barrier, on the one hand, to protect the burns from infection and calorico-nitrogen losses, on the other hand, the dermis acts as a mold to avoid hypertrophic cicatrices and retractions due to anarchic proliferation of fibroblasts.
Allographs are expensive and do not constitute an effective covering, because they are very often rejected, which causes additional shock for the patients.
The standard techniques of autograph are feasible only in the case of moderately extensive burns.
The progress of medico-surgical intensive care now makes it possible to keep burned patients alive with 80% of the cutaneous surface burned. This progress has resulted in increasing the need for skin or skin equivalent.
Numerous studies have been undertaken for this purpose.
Thus Green and O'Connor aimed at reconstituting the epidermis from keratocytes coming from the cells of the patient.
Bell and his team developed a skin equivalent consisting of a dermal layer of autologous fibroblasts and bovine collagen and an epidermal layer of autologous keratinocytes.
It should be noted that use of these two techniques requires a waiting time of twenty-one days (corresponding to the culture period of the cells) before being able to have artificial skin suitable for the patient.
A third technique was developed by Yannas and Burke (Jal of Biomedical Materials Research, Vol. 14, 65-81, 107-131, 511-528 (1980) and FR-A- 2 332 863).
The artificial skin proposed by these authors comprises two layers:
a dermal equivalent consisting of calf collagen and a glycosaminoglycans (GAG), chondroitin 6-sulfate, and crosslinked by glutaraldhyde;
a temporary pseudoepidermis of nonbiodegradable silicone resin limiting the exudative losses of the early phase and acting as a barrier to infectious agents, coming to cover the dermal layer.
This artificial skin is put in place after early excision of the third degree burned tissue (48 and 72 hours after burning). The collagen-GAG contains pores through which the endogenous neodermis in a way is "channeled." Thus, a "domestic" cicatricial tissue, very close to the normal dermis, is reconstituted, and at whose level the synthesis of collagen by fibroblasts does not occur in an anarchic manner unlike the tissue of usual granulation. When the dermal layer is revascularized (3 to 4 weeks), and when epidermal cells are deposited, the silicone membrane is removed. Then a extremely thin epidermal graft, as fillet 4 or 9, (taken from remaining donor areas) is put in place.
This process of cutaneous covering after early excision, already tested in the USA on more than two hundred burn patients, in five centers, proves to be particularly effective both in combating hydric and calorico-nitrogen exudative losses of the early phase and in preventing infection and to reduce the magnitude of the hypertrophic and retractile sequellae.
This compact, crosslinked matrix is colonized by fibroblasts of the wounded person which synthesize their own collagen and all the human proteins necessary for reconstitution of the connective tissue.
The degradation time of the artificial dermis, which is twenty four days, corresponds to the time of healing of the wound.
Thus channeled, the fibroblasts of the burned patent can take a good orientation, which avoid hydrotrophic cicatrices.
It remains, of course, to epidermize in a second period.
This material meets emergency needs. It can be sterilized and stored in large amounts.
This technique, advantageous though it may be, still has several drawbacks, both in regard to the dermis and the pseudoepedermis.
For the dermis:
The association of the collagen and glycosaminoglycans described by YANNAS and BURKE give composite materials in which the constituents are easily dissociable particularly because of the poverty of the collagen in ionizable basic functions. These composite materials are soluble in the liquids of the organism and should be crosslinked at the level of the collagen for their use in aqueous medium to be possible. However, the slight cohesion between crosslinked collagen and acid glycosaminoglycans is responsible for a progressive salting out of the GAGs each time an immersion in an aqueous solution is necessary (hydration, washing, neutralization, etc. . . ). Salting out, which is difficult to control, considerably modifies the final composition of the dermes.
To reduce its degradation, to increase its resistance, it is necessary to crosslink the collagen-GAG coprecipitate, either by chemical crosslinking by using glutaraldehyde, a product toxic to the organism, which requires, on the one hand, elimination of the excess glutaraldehyde and, on the other hand, control of its elimination of the finished product by physical crosslinking by the simultaneous action of heat and vacuum.
For the pseudoepidermis:
The pseudoepidermis of silicone resin is not biodegradable. Therefore it is necessary to remove it surgically before epidermizing.
Moreover, it cannot be used as a support of in vitro and in vivo culture.
The most recent technique is that proposed by WIDRA (EP-A-0 089 152 and EP-A-0 138 385), who proposes an artificial skin made from the association of anionic compounds derived from keratin and cationic biopolymers such as chitosan and collaqen.
These compounds can be applied in two staqes to form, IN SITU, a hydrogel membrane which forms a protective carapace and, at the end of cicatrization, hardens and falls off.